High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) is a technology that is capable of separating gas-phase ions at atmospheric pressure. Ions are introduced into an analyzer region, across which is applied a radio frequency (rf) waveform, such that the ions are subjected to alternating high and low strength electric fields. The applied waveform is asymmetric, comprising a repeating pattern including a high voltage component, V1, lasting for a short period of time t2 and a lower voltage component, V2, of opposite polarity, lasting a longer period of time t1. In particular, the waveform is synthesized such that the integrated voltage-time product, and thus the field-time product, during each complete cycle of the waveform is zero, for instance V1t2+V2t1=0; for example +2000 V for 10 μs followed by −1000 V for 20 μs. The peak voltage, V1, during the shorter duration, high voltage portion of the waveform is called the “dispersion voltage” or DV.
Ions are separated in FAIMS on the basis of a difference in the mobility of an ion at high field strength, Kh, relative to the mobility of the ion at low field strength, K. In other words, the ions are separated because of the compound dependent behavior of Kh as a function of the applied electric field strength. This field-dependent change in the mobility of the ions causes the ions to drift toward the walls of the analyzer region. In order to transmit an ion of interest through FAIMS, an appropriate direct current compensation voltage (CV) is applied to compensate for the drift of the ion of interest toward the analyzer walls. By varying the compensation voltage, different ions are selectively transmitted through the FAIMS device.
In general, the electrodes that define the analyzer region in a FAIMS device may be either flat or curved in shape, such as for instance parallel flat-plate electrodes or concentric-cylinder electrodes, respectively. The concentric-cylinder configuration provides higher sensitivity compared to the flat-plate configuration. This higher sensitivity is due to a two-dimensional atmospheric pressure ion-focusing effect that occurs in the analyzer region between the curved electrode surfaces of the concentric-cylinder electrodes. When no electrical voltages are applied to the cylinders the radial distribution of ions should be approximately uniform across the FAIMS analyzer. During application of DV and CV, however, the radial distribution of ions is not uniform across the annular space of the FAIMS analyzer region. With the application of an appropriate DV and CV for an ion of interest, those ions become focused into a band between the electrodes and the rate of loss of ions, as a result of collisions with the FAIMS electrodes, is reduced. The efficiency of transmission of the ions of interest through the analyzer region of a concentric-cylinder FAIMS is thereby improved as a result of this two-dimensional ion focusing effect.
On the other hand, the parallel flat-plate electrode configuration provides higher resolution. Resolution of a FAIMS device is defined in terms of the extent to which ions having similar mobility properties are separated under a set of predetermined operating conditions. Thus, a high-resolution FAIMS device transmits selectively a relatively small range of ion types having similar mobility properties, whereas a low-resolution FAIMS device transmits selectively a relatively large range of ion types having similar mobility properties. The resolution of FAIMS in the concentric-cylinder configuration is compromised relative to the resolution in the parallel flat-plate configuration because the concentric-cylinder configuration has the capability of focusing and trapping ions, as described above. This focusing action means that ions of a wider range of mobility characteristics are simultaneously focused in the analyzer region between the concentric cylinder-electrodes. Furthermore, a concentric-cylinder FAIMS device with narrow electrodes has the strongest focusing action, but the lowest resolution for separation of ions. As the radii of curvature of the cylinders are increased, the focusing action becomes weaker, and the ability of FAIMS to simultaneously focus ions of similar high-field mobility characteristics is similarly decreased. This means that the resolution of FAIMS increases as the radii of the electrodes are increased, with the parallel flat-plate configuration having the maximum attainable resolution.
Unfortunately, the sensitivity of a parallel flat-plate FAIMS device is low compared to the concentric-cylinder design. This is because as the ions transit through the analyzer region between the parallel flat-plates, diffusion and ion-ion repulsion forces, even though they are small, cause the ions to spread out in a direction along the width of the plates. In other words, the ions are introduced into the space between the flat-plate electrodes as an approximately collimated beam of ions, but rapidly spread out toward the edges of the electrodes to form a sheet of ions that travels through the analytical gap to the ion outlet. Accordingly, there has always been a trade-off to be made in the prior art between sensitivity and resolution when selecting an electrode configuration for a FAIMS device.
It would be desirable to provide an apparatus and method that overcome at least some of the above-mentioned limitations.